In general permanent-magnet-embedded electric motors, the rotor core, which is formed by stacking magnetic steel sheets that have been die-cut into a specific shape while they are being swaged, includes a plurality of magnet insertion holes into each of which a permanent magnet is inserted.
The relative position of the permanent magnet to the tooth tip in the rotating direction varies due to the rotation of the rotor. Magnetic flux inside the rotor is not inverted in its direction by 180 degrees but varies within a certain range according to variations in the current flowing in the winding wound around each tooth. When a rare earth magnet is used as an exemplary permanent magnet on the surface of which a current easily flows, variations in the magnetic flux inside the rotor result in eddy currents flowing on the surface of the permanent magnet. The eddy currents result in Joule heat in the permanent magnet and the Joule heat causes a loss in the electric motor. Furthermore, the Joule heat raises the temperature of the permanent magnet. Especially in the case of a rare earth magnet, coercivity drops, which may cause irreversible demagnetization.
As a method to mitigate the eddy currents generated on the surface of the permanent magnet, there is a method in which a permanent magnet per pole is divided into a plurality of pieces, i.e., a plurality of permanent magnets are inserted into a single magnet insertion hole. When a permanent magnet per pole is simply divided into a plurality of pieces, if the rotating-direction side surfaces of permanent magnets adjacent to each other among the permanent magnets inserted into the magnet insertion hole are in close contact with each other, the electrical resistance between the permanent magnets is reduced, thereby reducing the effect of mitigating the eddy currents.
In the rotor described in the following Patent Literature 1, the cross section of each magnet insertion hole viewed from the axial direction of the electric motor has a curved shape that is convex toward the radially inner side, and a plurality of permanent magnets are inserted into one magnet insertion hole such that they are arrayed in the rotating direction. This structure allows rotating-direction side surfaces of permanent magnets adjacent to each other among the permanent magnets inserted into the magnet insertion hole to be in contact with each other on the radially outer side while being apart from each other on the radially inner side. That is, the rotating-direction direction side surfaces of permanent magnets adjacent to each other are in contact with each other only on the radially outer side, thereby suppressing a decrease in electrical resistance between the permanent magnets.